Although the immunosuppressive drugs currently in use are effective in reducing the incidence of acute rejection after transplantation, they also put the patient at risk for life threatening infections and cancers1-3. The long term administration of immunosuppressive agents results in an increase in both morbidity and mortality. In addition, the long term cost of these agents represents a financial burden.[unreadable] [unreadable] Establishment of tolerance to a well-functioning transplant without nonspecific immunosuppressive drugs is a major goal of organ transplantation therapy4. The induction of recipient tolerance to the histocompatibility antigens of the organ donor could eliminate the need for long term administration of these nonspecific immunosuppressive drugs. This would have a major impact on the quality and quantity of life of patients with long term surviving organ grafts by reducing the immunologic and non-immunologic complications associated with long term immunosuppressive therapy. [unreadable] [unreadable] Induction of immunotolerance in animal heart transplant models has for the most part involved donor-based tolerance. One well established model is a single intravenous injection of donor splenocytes (25106 cells) plus a single intraperitoneal injection of RIB 52 (20mgkg) 21 days prior to the transplantation 5. RIB 52 is a nondepleting anti-rat CD4 monoclonal antibody that suppresses T helper 1 (Th1) cells thus induces robust allospecific tolerance. A recent study has shown that donor-specific blood transfusion induces the development of alloantigen-specific T helper 2 (Th2) cells in lymphoid tissues and in the graft. Adoptive transfer (altering the hosts immune system using lymphoid cells from another individual) of these ex vivo generated alloantigen-specific regulatory T cells induces immunotolerance in a rat transplant model 6. This method has also been used in bone marrow transplantation7 and can be the basis for recipient-based immunotolerance induction. In heart transplantation, recipient-based immunotolerance induction is clinically more applicable than donor-based since donor genotype is rarely known prior to transplant. To date recipient-based immunotolerance induction has not been tried in heart transplantation but data exists from the bone marrow transplant literature.[unreadable] [unreadable] In addition, although allograft tolerance has been achieved using a wide variety of donor-based immunologic interventions in laboratory animals, no reliable method of confirming tolerance following cardiac transplantation has been established. Standard of care in the field of solid organ transplantation remains initial treatment with triple immunosuppression followed by long term maintenance therapy with 1-3 immunosuppressive agents. One cannot withdraw drugs confidently even if the graft seems to be tolerant. [unreadable] [unreadable] This study will apply both genomics and proteomics within the same experiment with the goal of studying donor and recipient-based immunotolerance induction protocols and identifying protein and gene changes that could serve as possible candidate biomarkers of tolerance. Identifying laboratory methods that will permit safe and precise confirmation of immune tolerance in the transplant patient has the potential to improve outcome substantially.